A prior art centrifugal impeller for a premix blower for a gas fired water heater is made from metal and includes a back plate, a cover plate with an inlet opening and vanes connected to the back plate and the cover plate. The impeller is arranged inside a volute of the blower and is fixed to a shaft of a motor by way of the back plate. The inlet of the cover plate is located adjacent an inlet of the volute. In operation, a mixture of gas and air flows into the volute through the inlet, passes through the passages formed between the vanes, and flows out from an outlet of the volute. As the impeller is exposed to the mixture of gas and air, the impeller and the volute have antistatic properties to avoid a build up of static on the impeller, which otherwise may result in a static discharge igniting the gas and air mixture. The back plate, vanes and cover plate are made of the same material.
Japanese published Patent Application No. JP9-126185 discloses an impeller whose back plate, cover plate and vanes are all made of aluminum wherein two side edges of the vanes are respectively fixed to the cover plate and back plate. The weight of the impeller is high and its assembly process is complex. Also, accidental contact between the impeller and the volute may produce sparks with the potential to ignite the gas/air mixture. To solve these problems, an impeller with a back plate, cover plate and vanes all made of antistatic plastic was developed. However, its cost is many times the cost of the aluminum impeller.